In certain electrical machines, such as motors, generators and alternators, a shaft passing through one or more bearings supports a rotor assembly. In machines having a vertical shaft configuration, axial thrust force on bearings, without compensating forces, will include the rotor weight. In certain machines, such as flywheel motor/generator devices appearing in uninterruptible power systems, rotor weight may be substantial. U.S. Pat. No. 5,969,457, “Energy Storage Flywheel Apparatus and Methods” (the '457 patent), and U.S. Pat. No. 5,929,548, “High Inertia Inductor-Alternator” (the '548 patent), which this disclosure incorporates herein by reference in their entirety, describe such flywheel motor/generators. Magnitudes of thrust loads on bearings in such machines affect both power losses and bearing life. Reducing overall thrust force on bearings extends bearing life and reduces mechanical friction losses.
Magnetic unloading circuits may offset gravity or other thrust load forces on bearings that support a rotor. Magnetic unloading circuits may be “active,” “passive,” or “semi-active.” An active magnetic unloading circuit includes an electromagnet and a feedback controller for controlling unloading forces in response to feedback from a sensor (e.g., a force or displacement sensor). A passive unloading circuit, in contrast, uses no sensor feedback to control unloading forces. Instead, passive unloading circuits rely on forces that permanent magnets and/or electromagnets generate according to magnetic characteristics. “Semi-active” magnetic unloading circuits combine both active (e.g., feedback controlled electromagnets) and passive (e.g., permanent magnets) unloading elements.
Some rotating machines provide an “explicit” magnetic unloading circuit separate and distinct from the machine's primary magnetic circuit. An ideal explicit unloading circuit may be fully decoupled from a machine's primary magnetic circuit. In practice, however, there may be some magnetic coupling between unloading and primary magnetic circuits. Explicit magnetic unloading circuits are described in U.S. Pat. No. 4,444,444, entitled “Equipment for Storage of Energy Under Kinetic Form and Recovery Thereof in Electric Form and Method of Using Such Equipment” (the “'444 patent”), in U.S. Pat. No. 5,731,645, entitled “Integrated Motor/Generator/Flywheel Utilizing a Solid Steel Rotor” (the “'645 patent”), and U.S. Pat. No. 6,703,735, “Active Magnetic Thrust Bearing” (the “'735 patent) This disclosure incorporated fully and expressly the '444 patent, '645 patent, and '735 patent by reference, as though appearing here completely.
In some rotating machines, an “implicit” magnetic unloading circuit intentionally couples with the machine's primary magnetic circuit. Implicit circuits may use a sensor feedback circuit to maintain unloading within an appropriate range and may use a separate feedback circuit to control machine electrical performance. The '645 patent describes an example of an implicit magnetic unloading circuit and a homopolar inductor alternator device in which upper and lower field coils generate a combined primary air gap flux. The homopolar inductor alternator device controls primary field coil currents that flow in both the upper and lower coils in order to control the machine electrical performance (e.g., output voltage or torque). A load cell measurement of bearing thrust provides feedback control to generate a differential current that flows in the upper coil for maintaining a desired amount of thrust unloading.